Circuit Flow:Battery power for the "running lamps" enters the headlamp switch from the T/L CTSY fuse through the orange wire. Power splits through the headlamp switch out to the tail lamps (brown wire) and instrument lamps (green wire). The green wire of the instrument lamps circuit returns to the fuse block, routing through the INSTR LPS fuse, where it changes color to gray and runs to the instrument cluster and AC/heater control panel lamps. Of course, all lamps must be properly grounded to complete the circuit.
Testing:Probe both sides of the 20-amp T/L CTSY fuse using a
test light. If the fuse is hot, probe the orange wire at the headlamp switch.
[Bear in mind that even though a fuse may test good, if its connections to the fuse block are oxidized or dirty, current won't flow - so when testing a fuse, probe the terminals of the fuse "socket" with the fuse in place]. Inspect the plastic wire harness connector and headlamp switch for any evidence of heat damage (melting, distortion, or discoloration).
Rotate the headlamp switch knob counterclockwise and switch the headlamps ON. If the orange wire at the headlamp switch is hot, but the brown and green wires are dead, repair the harness connections to the switch and/or replace the switch, as needed. If the orange wire is dead, repair the "open" between the switch and T/L CTSY fuse. Reference:
Diagnosing dash lights that don't work.
Grounds:Grounds are subject to harsh environments. They are virtually ignored during routine maintenance. And, they are easily overlooked when troubleshooting. Over the years I've heard variations of, "Nah! They're too simple...." "Grounds don't have any moving parts! There's nothing to wear out!" "A ground couldn't possibly cause all these symptoms!" "Come on, what could go wrong with a ground!?!" Yet, they are a common source of headaches on trucks....
Now, is a great time to inspect and service all of the vehicle grounds: (1) battery-to-engine; (2) battery-to-right frame rail; (3) battery-to-right radiator support; (4) right rear cylinder head-to-firewall; (5) engine-to-left frame rail; (6) dash-to-left kick panel sheetmetal; and (7) rear lamps-to-bed sheetmetal, taped splices, and frame rail.
Turn Signals:If the turn signals aren't working, but the hazards are, replace the turn signal flasher (aka, "the other flasher").
I hope this has provided some insight.
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